1. Field of the Invention
The present invention relates to a liquid crystal display apparatus with a wide viewing angle and a method for producing the same, and also relates to a substrate.
2. Description of the Related Art
In a liquid crystal display (LCD), a liquid crystal layer including liquid crystal molecules is provided between a pair of substrates. When the alignment of the liquid crystal molecules is changed, the optical refractive index of the liquid crystal layer is also changed. By utilizing the change in the refractive index, the LCD performs the display. Accordingly, it is important that the liquid crystal molecules are arranged as regularly as possible in the initial state. In order to regularly arrange the liquid crystal molecules in the initial state, the surface conditions of the substrates which sandwich the liquid crystal layer should regulate the interactions between the liquid crystal molecules and the surfaces.
In the method for performing such a regulation which is currently the most widely used, material for a liquid crystal aligning film is applied to each of the surfaces of the substrates which face the liquid crystal layer. The applied material is dried and cured, so as to form the aligning film. Thereafter, the surface of the aligning film is rubbed.
There are two types of aligning films for regulating the alignment of the liquid crystal, i.e., an inorganic aligning film and an organic aligning film. Materials for the inorganic aligning film include oxides, inorganic silane, metals, and metal complexes, whereas materials for the organic aligning film include polyimides. A typical material for the liquid crystal aligning film which is currently employed is a polyimide resin. The polyimide resin is produced in the following manner. A polyamic acid, which is a precursor for all aromatic polyimides (all aromatic PI), is first applied to a substrate. Then, the substrate with the polyamic acid is heated so that an imidization reaction is caused to occur. As a result, the polyamic acid is converted into a polyimide resin. The reasons why the polyimide resin is widely used for the liquid crystal aligning film material are the concentration and the viscosity thereof can easily be adjusted since the polyamic acid has a good solubility, the polyimide resin has a good applicability, and that the thickness of the polyimide resin film can be easily controlled. The produced polyimide resin is more stable in terms of energy than the polyamic acid. Accordingly, when the substrate with the polyimide resin is cleaned by water, the reversible reaction will not occur.
The polyimide film which is formed on the substrate in the above-described manner is unidirectionally rubbed with a burnishing cloth or the like. Thus, the liquid crystal molecules can be aligned in the rubbing direction. The rubbing treatment is unidirectionally performed on the substrate, so that the tilt angles (i.e., pretilt angles) of the liquid crystal molecules in the liquid crystal layer which are in contact with the aligning film are all equal to each other. Accordingly, in each picture element constituting a dot as a unit of a matrix-type display pattern, all the pretilt angles are substantially equal to each other and are aligned in one direction.
In an active matrix type LCD which uses thin film transistors as switching elements connected to respective pixel electrodes constituting picture elements of the display pattern, that is, in a TFT-LCD, a construction of a twisted nematic (TN) type liquid crystal layer is adopted (an LCD of the TN mode). In such an LCD of the TN mode, the liquid crystal molecules between the pair of substrates are continuously twisted by 90.degree. along the direction perpendicular to the surfaces of the substrates. The viewing angle characteristics of the LCD define the optimal viewing angle direction and the viewing angle range depending on the directions of the liquid crystal molecules in the liquid crystal layer (the aligning directions and the tilt angles).
FIG. 27 shows a cross section of a picture element portion of an exemplary TN type LCD. The LCD is a TFT-LCD of an active matrix type. As is shown in FIG. 27, a liquid crystal layer 133 is sandwiched between substrates 131 and 132 which are provided so as to face each other. FIG. 26 is a plan view of the substrate 131 in FIG. 27. In the substrate 131, scanning lines 112 and signal lines 113 are formed so as to cross each other on a glass substrate 131a. In the vicinity of the crossings of the scanning lines 112 and the signal lines 113, thin film transistors (TFTs) 120 as nonlinear elements having switching function are formed. In areas defined by the scanning lines 112 and the signal lines 113, pixel electrodes 110 are formed, respectively. Each of the TFTs 120 includes a gate electrode 115 which is branched from a scanning line 112, a source electrode 116 which is branched from a signal line 113, and a drain electrode 117 for connecting the TFT 120 to a pixel electrode 110. The reference numeral 118 denotes an additional capacitance.
Over the above elements, as is shown in FIG. 27, an insulating protective film 131d and an aligning film 131e are formed in this order. In the other substrate 132, a color filter 132b, a transparent electrode 132c, an insulating protective film (not shown), and an aligning film 132e are formed on a glass substrate 132a in this order.
A liquid crystal molecule 133a in the liquid crystal layer 133 which is sandwiched by the above-mentioned substrates 131 and 132 is tilted as shown in FIG. 27, and the inclination represents the aligning direction of the liquid crystal. The substrates 131 and 132 are sealed at their ends by a resin or the like (not shown), and a peripheral circuit or the like for driving the liquid crystal is externally mounted. LCDs which are of types other than the active matrix type have the same construction as described above.
In the TN type LCD, since the liquid crystal molecule has anisotropy in the refractive index (birefringence), the contrast depends on the viewing angle at which a person (a viewer) views the LCD. In general, in the normally white mode of LCD in which light is transmitted during the no voltage application so as to perform a white display, when the LCD is viewed in the direction perpendicular to the surfaces of the substrates in a state where a voltage is applied across the electrodes formed in the respective substrates which sandwich the liquid crystal layer, the transmittance of light is decreased as the applied voltage value becomes high, as is shown by solid line L1 in FIG. 28. When the voltage value is saturated, the transmittance becomes substantially equal to zero. Accordingly, even when a much higher voltage is applied, the transmittance remains at substantially zero.
When the viewing angle is inclined from the direction perpendicular to the substrate face to the positive viewing angle direction, the applied voltage to transmittance characteristics are varied as is shown by the solid line L2 in FIG. 28. Specifically, as the applied voltage becomes high, the transmittance is decreased to some extent. When the applied voltage exceeds a specific value, the transmittance is increased. Then, the transmittance is gradually decreased. Therefore, when the viewing angle is inclined in the positive viewing angle direction, there occurs a phenomenon in that the black and the white (the negative and positive) of the image are reversed at a specific angle. This phenomenon occurs because the liquid crystal molecule in the liquid crystal layer has the tilt angle, and the refractive index is varied depending on the viewing angle. This phenomenon causes a serious problem for a person viewing the image.
Referring to FIG. 29, the problem will be described in detail. As is shown in FIG. 29(a), when the applied voltage is zero or a relatively lower voltage, the center molecule 133a of the liquid crystal layer is observed in the form of an ellipse by the viewer 137 positioned in the positive viewing angle direction. As the applied voltage is gradually increased, the center molecule 133a is moved in such a manner that the longer axis becomes aligned along the direction of the electric field, i.e., the direction perpendicular to the substrate face. Accordingly, the center molecule 133a is momentarily observed in the form of a circle by the viewer 137, as is shown in FIG. 29(b). As the voltage is further increased, the center molecule 133a becomes substantially in parallel to the electric field direction. As a result, the center molecule 133a is observed again in the form of an ellipse by the viewer 137, as is shown in FIG. 29(c). In this way, the reverse phenomenon occurs.
If the viewing angle is tilted in the negative viewing angle direction, the variation of the light transmittance is relatively small, as is shown by the solid line L3 in FIG. 28. As a result, the contrast is greatly degraded.
The above-described reverse phenomenon in the positive viewing angle direction and the degradation of contrast in the negative viewing angle direction cause serious problems for the viewer, and they result in doubts about the display properties of the LCD.
A technique for suppressing the reverse phenomenon in the TN mode LCD is described in, for example, Japanese Laid-Open Patent Publication No. 2-12. According to the technique, in the active matrix type LCD, a display electrode constituting a picture element is divided into an inner electrode and an outer electrode. By changing the conditions of the electric field applied to the liquid crystal molecules on the inner electrode side from those of the electric field applied to the liquid crystal molecules on the outer electrode side an attempt to improve the viewing angle characteristics was made.
However, the technique disadvantageously necessitates a variety of the electrode patterns, so that the production process and the driving method become complicated. Moreover, the resulting improvement of the viewing angle characteritics is not considered as being remarkable.
JAPAN DISPLAY '92, pages 591-594, and page 886 describe the following two methods. In one method, the surface of the aligning film is unidirectionally rubbed, and then a resist is deposited on a part of the aligning film. Then, the rubbing is performed in the direction reverse to the previous rubbing direction. Thereafter, the resist is removed. As a result, the aligning film is provided with different aligning conditions caused by the different rubbing directions between the aligning film surface covered with the resist and the aligning film surface not covered with the resist, so as to differentiate the pretilt angles. In the other method, polyimide aligning films made of different materials are juxtaposed and then they are subjected to the rubbing treatment. As a result, a plurality of pretilt angles are formed on the aligning films depending on the materials thereof.
However, if the resist is deposited on the surface of the aligning film, the alignment regulating property of the aligning film surface is greatly deteriorated. In the method for forming the polyimide aligning films of different materials, the patterning of the aligning films requires complicated process steps. For these reasons, the above methods are not practical.
In another attempt to eliminate the reverse phenomenon in the positive viewing angle direction and the contrast degradation in the negative viewing angle direction, a rectangular region 119, in which the aligning direction of liquid crystals is different from that in the other region, is formed in part of the picture element shown by a dotted line in FIG. 26. In more detail, both the regions of positive viewing angle and negative viewing angle are formed in one picture element, so that the contrast degradation in the negative viewing angle is compensated, and the reverse phenomenon in the positive viewing angle is suppressed.
However, in the above method, as the time elapses, the aligning condition of the rectangular region formed in part of the picture element may be absorbed by the aligning condition of the other region. In addition, in the boundary area between the rectangular region and the other region (an area indicated by dimension line C in FIG. 27), a disclination line occurs, i.e., the liquid crystals cannot be driven by the influence of both aligning conditions. This causes the contrast to be degraded.